1. Technical Field
The present application relates to an optical microphone for receiving an acoustic wave propagating through a gas such as the air and converting the received acoustic wave to an electric signal by using light.
2. Description of the Related Art
Microphones are known in the art as a device for receiving an acoustic wave and converting the acoustic wave into an electric signal. Many microphones, such as dynamic microphones and condenser microphones, include a diaphragm. With these microphones, a sound wave is received as the sound wave vibrates the diaphragm, and the vibration is taken out as an electric signal. A microphone of this type includes a mechanical vibrating section, such as a diaphragm, and properties of the mechanical vibrating section may possibly change as the microphone is used many times repeatedly. When detecting a very strong sound wave with a microphone, the mechanical vibrating section may possibly break.
In order to solve such problems of a conventional microphone having a mechanical vibrating section, Japanese Laid-Open Patent Publication No. 8-265262 (hereinafter, referred to as Patent Document No. 1) and Japanese Laid-Open Patent Publication No. 2009-085868 (hereinafter, referred to as Patent Document No. 2), for example, disclose optical microphones that do not have a mechanical vibrating section and that detect an acoustic wave by utilizing a light wave.
For example, Patent Document No. 1 discloses a method for detecting an acoustic wave by modulating light with an acoustic wave and detecting the modulated component of the light. Specifically, as shown in FIG. 36, a laser beam, which has been shaped using a light-outputting optical component 111, is made to act upon an acoustic wave 1 propagating through the air, thereby producing diffracted light. In this process, two diffracted light components in reverse phase are produced. After adjusting the diffracted light by a light-receiving optical component 112, only one of the two diffracted light components is received by an optical diode 113 and converted to an electric signal, thereby detecting the acoustic wave 1.
Patent Document No. 2 discloses a method for detecting an acoustic wave by propagating an acoustic wave through a medium and detecting changes in optical properties of the medium. As shown in FIG. 37, an acoustic wave 5 propagating through the air is taken in through an opening 201, and travels through an acoustic waveguide 202, of which at least a portion of the wall surface is formed by a photoacoustic propagation medium 203. The sound wave traveling through the acoustic waveguide 202 is taken in by the photoacoustic propagation medium 203 and propagates through the inside thereof. The photoacoustic propagation medium 203 undergoes a refractive index change as the sound wave propagates therethrough. The acoustic wave 5 is detected by extracting this refractive index change as an optical modulation by using a laser Doppler vibrometer 204. Patent Document No. 2 discloses that by using a dry silica gel as the photoacoustic propagation medium 203, the acoustic wave in the waveguide can be efficiently taken in into the inside of the photoacoustic propagation medium 203.